How to use mmap to point to STL type?

Question

I have a mmap typecast to a char pointer

char *ptr;

ptr = (char *)mmap(0, FILESIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

This was my earlier code. But now I want to use a map instead of char * as the requirements changed.

Now, my map is declared as map < int, string > i_s_map;

How do I change my mmap call to point to the map?

Answer 1

You don't want to store STL containers in shared memory, at least not share them. The reason is that they rely heavily on heap allocation, so out-of-the-box std::map will hold pointers from virtual address space of a different process.

Take a look at boost::interprocess for a way to deal with this situation in C++.

Answer 2

If you want to create a map object in the memory returned by mmap use placement new.

map<int,string> *i_s_map = new(ptr) map<int,string>();

That will create the map object itself in the memory. In order to get the elements inside the map into the memory, you will need to create a custom allocator to keep the data in the memory. You can use the boost interprocess library for some allocators which work inside shared memory.

http://www.boost.org/doc/libs/1_42_0/doc/html/interprocess/allocators_containers.html#interprocess.allocators_containers.allocator_introduction